1. Technical Field
The present disclosure relates to a semiconductor device integrating a voltage divider and to a process for manufacturing a semiconductor device.
2. Discussion of the Related Art
Known to the art are particular types of semiconductor devices integrating resistors for medium-to-high voltage applications, which are capable of withstanding voltages on the order of some hundreds of volts or even higher.
For example, in power supplies and/or battery chargers of many portable electronic devices, semiconductor voltage dividers are used for detecting the grid voltage, as well as for performing further functions. In these cases, the resistors should withstand voltages of 500-800 V.
Some types of resistors suitable for medium and high voltages comprise polysilicon strips, which extend along a spiral path and are provided on a semiconductor substrate, with interposition of a dielectric layer. The radially inner terminal of the spiral resistor is generally coupled to a high-voltage line, whereas the radially outer terminal is coupled to a low-voltage line. If need be, in order to provide a voltage divider, a tap is formed with a contact towards the outside at an intermediate point of the resistive spiral.
Resistors of this type exploit very well the available area and do not present corners, which could easily lead to conditions of breakdown, given the voltages involved. However, known high-voltage resistors suffer from some limitations.
In the first place, the resistors are made of doped polysilicon and are subject to voltage differences with respect to the surrounding regions, both towards the substrate, and, in the opposite direction, towards the package or some protection structures, or else may be exposed to interfaces and/or dielectrics with trapped electrical charge. The voltage differences that are created cause depletion regions at the interface of the polysilicon forming the resistors and reduce the conduction section, increasing the resistivity. Moreover, the extension of the depletion regions and the corresponding influence on the resistance depend upon the voltage applied and on the local bias conditions and hence the effects are variable, moreover in a non-linear way, along the resistor. According to the bias conditions, not only may the value of overall resistance vary, but the division ratio of a voltage divider is not constant either. For example, the division ratio may vary by several percentage points when the voltage across the divider varies between 0 V and 500 V, whereas a more contained variation would be desirable.
In order to limit the effects of the voltage difference with respect to the substrate, it has been proposed to integrate a power component, for example a MOSFET, with cylindrical symmetry concentric to the spiral resistor. The power component has a radial-conduction diffusion region in which the distribution of equipotential lines is substantially equal to the voltage drop along the resistor and prevents depletion of the polysilicon. This solution has proven effective in limiting the effects due to the substrate, but has no effect as regards the package and the overlying protection structures.
Other limitations depend upon the presence of metallization regions in the proximity of parts of the resistor (in particular, at the high-voltage and low-voltage contacts and at the intermediate tap of a voltage divider). During the manufacturing process, in fact, steps of annealing in a forming gas are carried out, during which the resistivity of the polysilicon is modified. The metallization regions provide a shielding function in regard to the underlying polysilicon regions so that the action of the forming gas during annealing is attenuated. The effects of annealing in these conditions are not uniform and have a low predictability.
It would be desirable to provide a semiconductor device and a process for manufacturing a semiconductor device that will reduce the described limitations.